As shown in FIG. 1, a filtration device 10 of the prior art comprises a shell 11, a filtering screen 12, an inlet 15, an outlet 16, an impurities discharging tube 17, and a valve 18. The filtering screen 12 is tubular in shape and is housed in the shell 11 shch that the interior of the shell 11 is divided into a first compartment 13 and a second compartment 14. The inlet 15 is so disposed that it is in communication with the first compartment 13. The outlet 16 is mounted to the side of the shell 11 such that the outlet 16 is in communication with the second compartment 14. The impurities discharging tube 17 is mounted at the lower portion of the shell 11 such that the impurities discharging tube 17 is in communication with the first compartment 13. The valve 18 is fastened with the impurities discharging tube 17. In operation, a fluid to be filtered is injected into the shell 11 via the inlet 15 and is then filtered by the filtering screen 12. The filtered fluid is let out via the outlet 16. The filtered fluid can be filtered repeatedly if necessary. In the filtering process, the particles having diameters greater than the diameter of meshes of the filtering screen 12 are kept in the first compartment 13 such that most of the particles are adhered to the inner wall surface of the filtering screen 12, thereby obstructing the filtration process and slowing down the filtration rate. As the valve 18 is opened, the impurities so collected in the first compartment 13 are discharged via the impurities discharging tube 17.
The prior art filtration device 10 described above is defective in design in that only the impurities suspended in the first compartment 13 can be discharged, and that the impurities adhered to the inner wall surface of the filtering screen 12 can not be washed away effectively. The filtering screen 12 can be dismounted and washed; nevertheless the entire filtration device 10 must be shut down to allow the filtering screen 12 to be removed and washed.